Etching electrodes

ABSTRACT

A SHADOW MASK USED IN SCREENING A COLOR PICTURE TUBE IS RE-ETCHED IN A MULTISTATION APPARATUS TO HAVE HOLES OF LARGER SIZE THAN THE PHOSPHOR DOTS. A DENSITOMETER, POSITIONED IN AN INTERMEDIATE ETCHING STAGE, MEASURES THE TRANSMISSIVITY OF THE MASK TO LIGHT AND CONTROLS THE ETCHING PROCESS TO ACHIEVE THE DESIRED HOLE SIZE. AN ETCHANT IS SPRAYED INTO THAT STAGE TO MAINTAIN SUBSTANTIALLY CONSTANT   ATMOPSHERIC CONDITIONS THEREIN AND THEREBY AVOID CHANGES IN CALIBRATIN OF THE DENSITOMETER THAT ARE ATTRIBUTABLE TO VARYING RESPONSE OF THE SHADOW MASK MATERIAL TO THE ETCHANT OR TO OPERATING CONDITIONS WHICH OTHERWISE LEAD TO INCORRECT FINAL HOLE SIZE IN THE MASK.

May 30, 1972 L. DIETCH 3,665,579

ETCHING ELECTRODES Filed June 15, 1970 r J29 Densitometer F 1 Control 0 O ll IO M r k rv Oxide r Rinse ST I 1st. 2ndfl 3rd. 4th t Decorbomze r'pper Stoge Stoge Stage Stage I Blocken R'nse 1 0.1. Rinse no. 2 Progrommer 0 Control From Etchont Source 30 Inventor Leonord Dietoh 1 st. ZnEfBrZ trh m By 3 Attorney United States Patent 3,666,579 ETCHING ELECTRODES Leonard Dietch, Skokie, Ill., assignor to Zenith Radio Corporation, Chicago, Ill. Filed June 15, 1970, Ser. No. 46,354 Int. Cl. C23f l/02 US. Cl. 156-16 8 Claims ABSTRACT OF THE DISCLOSURE A shadow mask used in screening a color picture tube is re-etched in a multistation apparatus to have holes of larger size than the phosphor dots. A densitometer, positioned in an intermediate etching stage, measures the transmissivity of the mask to light and controls the etching process to achieve the desired hole size. An etchant is sprayed into that stage to maintain substantially constant atmospheric conditions therein and thereby avoid changes in calibration of the densitometer that are attributable to varying response of the shadow mask material to the etchant or to operating conditions which otherwise lead to incorrect final hole size in the mask.

RELATED APPLICATION The subject invention is related to and is a further improvement in the process of etching a shadow mask described and claimed in copending application Ser. No. 6,619, filed J an. 28, 1970 in the name of Martin L. Lerner and assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION The Lerner application describes an etching process which, while subject to general utility, is especially suited for conducting what has come to be known as etch back or re-etch.

Etch back is a term that evolved in connection with the manufacture of color picture tubes characterized by the fact that the phosphor deposits of its screen are smaller in size than the apertures of the color selection electrode or shadow mask with which they are associated. One attractive method of screening such a tube features forming the shadow mask initially to have holes dimensioned as required for utilizing the mask in photographic printing of the screen. After screening has been completed, the holes of the mask are enlarged by further etching and hence the further etching process is referred to as etch back or re-etching of the mask.

It was thought initially that etch back could be accomplished satisfactorily simply by spraying an etchant over the mask for an accurately controlled period of time but experienced proves this not to be so. As explained in the Lerner application, it is necessary to determine the response of the particular mask in process to the etchant and from the measured response compute the etch back time required to enlarge the holes to a desired predetermined size. The measurement is achieved by first subjecting the shadow mask to an etch for a given period of time and measuring, by means of a densitometer, the increase in transmissivity of the mask as a result of the etching treatment. The densitometer lends itself to direct calibration and develops a control effect used for the control of succeeding etching stages as required to arrive at the desired final hole size of the mask.

Conditions may be encountered, however, which cause the calibration of the densitometer to vary and, should that occur, inaccuracies are introduced into the etch back process. Inaccuracies attributable to transitory variations in calibration of the densitometer are avoided or greatly minimized by the subject invention.

3,666,579 Patented May 30, 1972 Accordingly, it is an object of the invention to improve etching processes of the type under consideration.

It is a specific object of the invention to achieve further improvement in the accuracy of the etch back process of shadow mask fabrication.

It is still another particular object of the invention to improve the accuracy of a multistage etching apparatus in etching apertures in an electrode to a desired predetermined size.

SUMMARY OF THE INVENTION The invention is an improvement in the process of etching apertures to a predetermined size in an electrode subject to attack by a particular etchant. The process to be'. improved upon includes the steps of etching the electrode in one work station by spraying with the etchant for a preselected period of time and at the end of that period moving the electrode to the next succeeding work station. In the next process step the transmissivity of the electrode to light is measured to determine the response of the electrode to the etchant spray and to determine the additional etching, if any, that is to be conducted in the succeeding work station to achieve the desired aperture size. This general process is improved by establishing a reference atmospheric condition in'the aforesaid succeeding work station for the duration of the step of measuring transmissivity.

More specifically, in mass production a series of electrodes are delivered one after the other to the etching apparatus. A densitometer is located at an intermediate one of the etching stages to make the necessary measurement of transmissivity and in each cycle of the apparatus a quantity of etchant is sprayed into that stage so that the atmosphere is essentially the same at the time the transmissivity measurement is made whether that particular stage has, in fact, etched the preceding electrode passing through the apparatus or not.

BRIEF DESCRIPTION OF THE DRAWING The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is a schematic representation of an etch back apparatus with which the subject invention may be practiced;

FIG. 2 is a detail of one stage of the apparatus of FIG. 1 showing the utilization of the subject invention; and

FIG. 3 includes a set of curves used in explaining the etching process and apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more particularly to the drawing, the arrangement of FIG. 1 is for re-etching electrodes, specifically shadow masks of color picture tubes, so that the apertures thereof are enlarged to a predetermined size. The apparatus is for mass production in which a conveyor presents a succession of such electrodes or masks seriatim to the various stages of the apparatus. The first stage 10 is an oxide stripper that is required because in photoscreening, in which the shadow mask is employed to determine the size and location of the families of phosphor dots, it is common practice to blacken the mask or pro vide it with an oxide coating so that it has radiation properties analogous to those of a black body. The oxide, however, resists the attack of an etchant that otherwise would be effective in opening or enlarging the holes of the mask; The mask usually is formed of cold rolled steel and a suitable etchant is ferric chloride. In order toavoid attenuating the effect of the etchant, the mask is treated'at the first work station with hydrochloric'acid and a detergent which strip the oxide coating after which'the mask is rinsed with water.

The next portion 11 of the apparatus is'a four-stage etching arrangement and each mask is advanced, step by step, through each of the four stages or work stations and provided with a dwell or treatment period of 10 to 12 seconds. It is convenient to have the same process time available at each stage and to have a conventional programmer or timer control the conveyor as required to present each mask to the various ones of the four stages. It is also convenient from the standpoint of process'control to have each of the four etching stages fed from a common etchant supply so that such parameters as concentration and temperature are the same at each stage.

FIG. 2 illustrates a preferred posture for a shadow mask in an etching stage. As there indicated, the apertured part 20 of the mask faces downwardly and the annexed frame 21 faces upwardly. The frame has the customary leaf springs which engage a fixture 22 of the conveyor, equipped with coasters 23 that ride along rails to advance the work in process from one stage to the next. It is also preferred, as indicated in FIG. 2, to have the etchant sprayed vertically upwardly against the lowermost face of mask 20, the etchant being delivered through a common manifold 24 to an array 25 of spray heads dimensioned and spaced to achieve a substantially uniform application of etchant to the surface of mask 20. 7

Advantages of this manner ofprocessing the mask are set forth in an application Ser. No. 45,681, filed June 12, 1970 in the name of Martin L. Lerner and assigned to the assignee of the present invention.

Each of the four etching stations is equipped with sliding doors (not shown) that may close over entrance and exit ports so that the stage is, in effect, a closed chamber when etching is being conducted. If the stageof FIG. 2 is assumed to be the third stage of arrangement 11, it includes a densitometer control system 29 which measures the response of a mask in process to the etchant in the first and second stages inorder to determine what additional etching, if any, is to be carried out in the third and I fourth stages of the etching apparatus. The densitometer measured. This is accomplished by a light source -26 which projects a beam of light 27 through the centralpon tion of the mask instantaneously in the third stage to a photocell or light pickup device 28 secured in the top Wall of the etching chamber directly across from-light source 26. It is preferable that light beam 27 have a large cross section compared with the size of the holes inmask 20. An acceptable size is a beam of about one inch diameter. The photocell'28 is the input to the densitometer which is employed so that the measured transmissivity of the mask at the third stage determines the extent, if any, to which additional etching is accomplished in the thirdand fourth stages of unit lL'The control is easily exerted by the output from the desitometer, as indicated in FIG. 2, through an. electrically actuable valve 30 having an associated solenoid control 31 in the supply to manifold24, The control may be duplicated at stages 3 and 4. j

If the mask instantaneously at the third stage responds to the influence of the etchant in the manner indicated by curve A of FIG. 3, it is apparent that the mask upon reaching the third stage has aperture sizes less than the minimum value of the tolerance range indicated by" the horizontal lines min and max of FIG. 3 and, therefore, the densitometer permits etching to take place in the third stage for a controlled period of time to attain an acceptable hole size in the mask. Since the apertures of the mask attain a desired size within the etching time allowable in stage 3, the densitometer will permit no etching to take place in stage 4 and will permit only a sufiicient amount of the available etching time to be used in stage 3 as necessary to achieve the desired aperture size at which time etching is interrupted in the third stage.

Where the response of the mask to the etchant is that indicated in curve B, the densitorneter permits the full etching time to take place in the third stage and most of theetching time to be used in the fourth stages. At the other extreme is the condition of curve C which indicates that the apertures have attained the desired final size at the end of the etching time in stage 2. For this condition, the densitometer prevents etching in either of stages 3 and 4. All of this description is merely duplicative of that of Lerner application Serial No. 6,619 and constitutes no part of the present invention which is an improvement over the Lerner process.

If it be assumed that the stock from which all of the masks have been constructed so responds to the etchant that the final hole size is always achieved in the fourth stage, it will be apparent that essentially identical atmospheric conditions are presented in stage 3 each time the transmissivity of any one of the succession of masks is to be measured. That is to say, there will always be the samedensity of etch mist in the chamber presenting a constant attenuation of light in its travel from source 26 to photocell 28. Since the conditions are the same at each of the measuring times, the calibration of the densitometer remains fixed and the system proceeds as described above. But certain conditions may be encountered which will adversely effect calibration of the densitometer and the r final hole size of at least some of the masks of the series being processed.

Consider, for example, the case in which a mask does not happen to be carried by one particular fixture of the conveyor. When that fixture advances to the third stage i and the transmissivity measurement is made, maximum that the conveyor fixture carrying no mask is within the third stage. Therefore, the spray orv mist of etchant otherwise generated when etching takes place in the third stage dissipates so that when the next succeding conveyor fixture moves into the third stage and carries a mask with i it, there is distinctly less mist and less attenuation'in the light path from source 26 to photocell 28 than is normally the case. This means that more light registers on the photocell than would be the case in the presence of the normal conditions of mist or spray of the etchant in the third stage and an incorrect measure of transmissivity results. This is avoided in accordance with the improvement of the present invention by establishing a reference atmospheric condition in the third stage for the duration of each measuring step. More specifically, the desired atmospheric condition is assured by the step of spraying a' quantity of etchant into the third stage at least after each occasion in which no etching has taken place in that stage for the preceding processing interval. As a practical matter, it is more simple to arrange that a quantity of etchant be sprayed into the chamber, creating the desired atmosphere, in the etching cycle of every step advance of the conveyor. A separate spray head could be provided in the third stage for that purpose but again for simplicity a programmer 40' actuates solenoid 31 to inject etchant through the array of spray heads 25 at controlled periods of time in each operating cycle of the etching arrangement. It has been found acceptable to have programmer 40 inject etchant through spray heads 25 over a period of about one second following the close of the -12 second operating interval assigned to the third etching stage and as, or after, a mask is moved out of the third stage. This assures that when a fixture of the conveyor moves out of the third stage and the next succeeding fixture enters that stage, the existing atmospheric conditions will always be essentially the same. This avoids undesired variations in calibration of the densitometer and assures more precise control of the re-etching or etch back system. Of course, instead of injecting etchant at the conclusion of the operating time allotted to the third stage, it may be done at the start of that interval. It is preferred to inject the etchant at the close of that interval, however, to give the mist a desired time to distribute throughout the chamber and its distribution may be aided by a blower (not shown) which is usually provided, as described in the Lerner application, to blow air or other suitable gas across the conveyor to avoid turbulence of the miniscus-type lens that forms in the apertures of the mask in the etching process.

Under the influence of programmer 40, therefore, there is an injection of etchant into the third etching stage for each cycle of the re-etch apparatus. This occurs whether etching takes place in the third stage during a particular cycle or not. As explained, if the conveyor does not present a mask to the third stage, or if the mask presented to that stage has the response of curve C to the etchant so that the desired hole size has been attained at the end of the etching step conducted in the second stage, no etching is carried out in the third stage for that particular cycle of the apparatus. And yet programmer 40 causes etchant to be injected into the third stage at the end of the process time available for etching in that stage even though etching does not, in fact, occur therein the circumstances recited. Nevertheless, a mist of etchant is established so that a valid measure of transmissivity may be made when the conveyor moves the next mask into the third stage for measurement.

Each re-etched mask upon leaving the fourth stage of apparatus 11 enters a stage 12 where it is rinsed, decarbonized, blackened and rinsed again as described in the Lerner application Ser. No. 6,619.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. In the process of etching apertures to a predetermined size in an electrode subject to attack by a particular etchant, which process includes the steps of:

(a) etching said electrode in one work station by spraying with said etchant for a preselected period of time,

(b) at the end of said period moving said electrode to a succeeding work station, and

,( c) measuring at said succeeding work station the transmissivity of said electrode to light to determine the response of said electrode to said etchant spray and to determine the amount of spray etching to be conducted at said succeeding work station to achieve said predetermined aperture size, the improvement which comprises establishing a reference atmospheric condition in said succeeding work station for the duration of said step of measuring the transmissivity of said electrode.

2. The improvement in accordance with claim 1 in which said reference atmospheric condition is established by spraying a quantity of said etchant into said succeeding work station immediately prior to said measuring step.

3. The improvement in accordance with claim 2 in which said succeeding work station is controlled to spray etch said electrode for a given maximum etching interval;

and in which said quantity of etchant is sprayed into said succeeding work station for another interval that is short compared with said maximum interval.

4. The improvement in accordance with claim 3 in which etching is conducted in said succeeding work station by spraying said etchant through an array of spray nozzles for one time interval;

and in which said reference atmospheric condition is established by spraying said quantity of etchant into said succeeding work station through the same array of spray nozzles for a different time interval.

5. The improvement in accordance with claim 4 in which said quantity of etchant is sprayed into said succeed ing work station after said electrode is in position therein and prior to the start of said measuring step.

6. In the process of etching apertures to the same predetermined size in each of a succession of electrodes, which process includes for each said electrode, the steps of:

(a) etching said electrode in one work station by spraying with an etchant for a preselected period of time,

(b) at the end of said period moving said electrode to a succeeding work station,

(0) measuring at said succeeding work station the transmissivity of said electrode to light to determine the response of said electrode to said etchant spray and to determine the amount of spray etching to be conducted at said succeeding work station to achieve said predetermined aperture size, and

(d) further etching said electrode in said succeeding work station in response to ,a determination by said measuring step that the apertures of said electrode are less than said predetermined size.

the improvement which comprises spraying a quantity of said etchant into said succeeding work station prior to conducting said measuring step to establish a reference atmospheric condition for the duration of said measuring step.

7. The improvement in accordance with claim 6 in which said quantity of etchant is sprayed into said succeeding work station immediately after each of said electrodes is moved into position therein and prior to said measuring step.

8. The improvement in accordance with claim 6 in which said quantity of etchant is sprayed into said succeeding work station immediately after each of said electrodes is moved out of said succeeding work station and prior to the positioning of the next one of said electrodes in said succeeding work station.

References Cited UNITED STATES PATENTS 2,875,141 2/1959 Noyce .15617UX 3,401,068 9/1968 Benton 1563 3,553,052 1/1971 Jubb 1567 X WILLIAM A. POWELL, Primary Examiner US. Cl. X.R. 156--345 

